Field of the Invention
The present invention relates to a hybrid heating furnace system for hot stamping, in which a high-frequency heating furnace is combined with an electric furnace (or a gas furnace).
Description of the Related Art
In vehicle manufacturing fields, there has been a focus on a decrease in weight and an increase in strength. Recently, hot stamping technology has been located at the center of this focus. The hot stamping technology was proposed by Norrbottens Jarnverk AB in Sweden in the early 1970s. In GB Patent No. 1490535 issued to this company, the hot stamping technology is disclosed in detail.
Referring to FIG. 1, the hot stamping technology is characterized by manufacturing a steel plate part 1b having high strength of 1500 MPa by heating a steel plate 1 in a heating furnace 10 at high temperature of Ac3 or more, and forming and heat-treating the heated steel plate in a press 20. This hot stamping process is called hot forming, hot pressing, or the like. As a material for the hot stamping, a so-called boron steel is used which contains carbon of about 0.2 wt % and uses manganese (Mn) and boron (B) as elements for improving heat treatment performance.
This hot stamping technology has advantages in that it simultaneously carries out formation and heat treatment to thereby provide excellent productivity, in that it forms a steel plate at high temperature to thereby improve formability and dimensional precision, and in that it remarkably reduces spring-back or delayed fracture that becomes an issue, particularly, in high-strength parts. In contrast, the hot stamping technology has disadvantages in that it cannot avoid surface oxidation of the steel plate due to a high-temperature process, and that it must perform a separate descaling process 30 on formed products as illustrated in FIG. 1. In order to remove this descaling process, there has been a proposal for aluminized steel sheets, etc. which are available from Arcelor or Nippon Steel.
Meanwhile, in terms of application of the hot stamping technology, heating furnaces that have mainly been used for a long time are electric furnaces. The steel plate for the hot stamping must be completely austenitized by heating at a temperature between 880° C. and 950° C. above Ac3. For the austenitization, the steel plate, being 1.2 mm thick, requires between 12 minutes and 17 minutes when the electric furnace is used. In this manner, the heating furnaces such as electric furnaces or gas furnaces increase heating time, and thus cause a decrease in process speed and an increase in production cost. Furthermore, the heating furnaces have a considerable length ranging from 23 m to 30 m, and thus have no choice but to have a huge volume. These large facilities increase various expenses.
Currently, high-frequency induction heating is used for local strengthening of body parts such as center pillars. This high-frequency induction heating can heat the steel plate to a temperature of 1000° C. or more within several seconds. As such, in the case where high-frequency induction heating is applied to hot stamping technology, this can make the heating furnaces small, and reduce heating time and cost of the steel plate. However, in the case of the heating furnace based on the high-frequency induction heating (hereinafter, referred to as “high-frequency furnace”), the heating furnace has a problem of a sharp increase in temperature or deformation during transferring the steel plate. For this reason, the heating furnace has been merely used for heat treatment of somewhat thick parts rather than thin parts.